1. Field of the Invention
The present invention relates to a tape cassette including an upper half and a lower half and, more particularly, a tape cassette having a partially reduced wall thickness without changing parting line (PL) of molds to be used.
2. Prior Art and Problem to be Solved
As shown in FIG. 1, the conventional tape cassette is composed of a housing including an upper half 2, a lower half 3 and a guard panel 1 as main components. Trials have been made in production of this type of tape cassette to reduce the weight of the tape cassette by reducing the thickness of a major bottom wall portion of the lower half 3, which leads to reduction in the time required for its molding operation as well as reduction of the manufacturing cost.
However, this type of cassette had an inherent drawback. This will be explained in reference to FIG. 2. First of all, solidification of a plastic material used for molding such cassette will be explained. FIG. 2 illustrates a cross section 4 of a molded article having a thinner wall portion 6 and thicker wall portion 7 having a transition area between them. It is well known in the art that, when cooled after injection molding of a plastic material, the thinner wall portion 6 is solidified faster than the thicker wall portion 7. During the course of solidification, the thinner wall portion 6 draws a portion of unsolidified plastic material from the adjacent thicker wall portion 7, with the result that surface layer 5 of the wall portion 7 is recessed as shown in FIG. 2, to form an unsightly shrinkage mark. Thus, the difference in the solidification time gives rise to shrinkage mark.
With a tape cassette having a lower half 3 having a thin major wall portion 10 in the form of an eye mask and thick wall portions in the other portions such as those indicated at A and B in FIG. 3(a), shrinkage marks 12 and 13 are necessarily formed as shown in FIG. 3(c) on the outer surface of the lower half in the areas as shown corresponding to a groove 8 and a triangular portions 11 of FIG. 3(a).
In order to solve this shrinkage mark problem, the applicant tried to simply make the portion groove 8 and triangular portion 9 thinner than normal and this measure was proved effective. However, although the problem of shrinkage mark is solved, a new problem arises due to the fact that the parting line PL of molds for these portions 8 and 9 does not conform to that for the thick portions in the areas A and B and thus the lateral surfaces of the molds are engaged with each other to rub off the engaged areas of the lateral surfaces, thereby causing the problem of reducing the life of the molds. More specifically, mating faces of the molds 14 and 15, which define a cavity 16 for forming a bottom wall of the groove 8 or the triangular portion 9, conforms to the parting line PL prior to the modification as shown in FIG. 4. On the other hand, mating faces of the modified molds 14' and 15', which define a cavity 16' for forming a bottom wall of the groove 8 or the triangular portion 9, include a projection different from the parting line PL for reducing the thickness of the bottom wall and thus the molds 14' and 15' include sliding contact portions 17 on their lateral surfaces as shown in FIG. 5, resulting in attrition or scraping of the contact portions 17.